APPLICANT'S ABSTRACT: The major objective of this work is to clone and characterize at least one additional mammalian (human/rat) gene encoding a dopamine D1-like receptor subtype which may subserve some of cocaine's psychostimulant actions. Of cocaine's many biochemical effects in the brain, it is generally accepted that via the direct blockade of dopamine uptake and concomitant increase of synaptic dopamine, the initial cellular actions of cocaine result in the stimulation of multiple dopamine receptors, particularly of the D1 subtype, to elicit psychomotor behavior associated with euphoria, self reward and addiction. While we were first to clone the dopamine D1 receptor and a human D1 receptor variant, the D5 receptor, recent pharmacological and behavioral data indicates the existence of yet additional dopamine D1-like receptor subtypes. Indeed, we now provide the first direct support for the contention that at least three distinct dopamine D1 receptor genes exist in number of vertebrate species, such as Xenopus laveis and Gallus domesticus. Thus, in addition to D1/D1A and D5/D1B receptor genes both the frog and chicken express a third D1 receptor subtype, which we term D1C and DlD, respectively. The novel frog D1C and chicken DlD receptors are each distinguishable from DlA and DlB receptor genes and from each other on the basis of their primary structure, mRNA distribution and unique pharmacological profiles. Our specific aims are, therefore, to: 1) Clone the mammalian homolog[s] of either the frog D1C and/or chicken D1D receptor genes. 2) Characterize the pharmacological signatures of novel subtype specific D1 like receptors and identify high affinity, subtype selective agonists and antagonists. 3) Define, in co-expression studies, whether receptors are functionally differentiated in terms of: (1) coupling to subtype specific G-proteins and effector systems and, (2) agonist desensitization profiles. 4) Using receptor chimeras constructed from distinct regions of cloned mammalian and vertebrate D1-like receptors define the structural motifs and amino acid residues that contribute to the unique pharmacological/signal transduction and agonist desensitization profiles exhibited by these novel receptor subtypes. We expect that data obtained from these studies will: allow for the development of selective and subtype specific dopamine D1 receptor compounds that may be of therapeutic value in combating cocaine's abuse potential and; provide tools [genes/antibodies/ligands/cell lines] and model systems to assess at the genetic, molecular, and biochemical level, the full impact of chronic cocaine use on the D1 receptor system in the brain. From a broader scientific perspective, our studies of multiple dopamine D1 receptors may contribute to pharmacological approaches for treatment of negative symptoms in schizophrenia, waning drug effects in Parkinson's disease, alleviation of extrapyramidal disorders following chronic neuroleptic treatment and in understanding cellular correlates of dopamine action on multiple D1 receptor systems.